1. Field of Invention
The present invention relates to a monitoring device in semiconductor manufacturing. More particularly, the present invention relates to a phase error pattern for monitoring the phase error of an alternating phase shift mask (Alt-PSM).
2. Description of Related Art
Following the rapid increase in level of integration, line width of semiconductor devices is often smaller than the wavelength of light source used in photolithographic process. To obtain better pattern resolution, a phase shift mask is routinely used. The earlier type of phase shift mask includes the alternating phase shift mask generally used for producing a plurality of mutually parallel gate lines.
FIG. 1 is a schematic cross-sectional view of a conventional alternating phase shift mask. As shown in FIG. 1, the alternating phase shift mask 100 includes non-transparent regions 110 covered with chromium material and transparent regions 120 and phase shift regions 130 alternating between the non-transparent regions 110. The phase shift regions 130 are 180xc2x0 out of phase with the transparent regions 120. Hence, light coming from the photoresist layer (not shown) corresponding in position to the non-transparent regions 110 is canceled resulting in a better exposure contrast.
Although a number of methods are available for producing phase shift regions 130 whose phase differs from the transparent regions 120 by 180xc2x0, actual phase difference between the two regions frequently deviates from the desired 180xc2x0 because deposition and etching are difficult to control. For example, for a positive photoresist, with reference to both FIGS. 1 and 2, if the focal point of the light source deviates (that is, defocused), the photoresist pattern 210 may shift to a position closer to the phase shift region 130. The amount of shifting s for a pair of photoresist patterns 210 flanking the same phase shift region 130 is identical but in opposite direction.
The shifting of photoresist pattern 210 often leads to an error in the final pattern position on a wafer. Thus, deviation of photoresist pattern is preferably measured with precision so that phase errors in the mask can be deduced as a reference for correcting the photomask. Unfortunately, the overlay error analyzers used in existing inline monitoring system are mostly optical and wavelength of the light used in detecting position error in photoresist pattern is much greater than the line width of the photoresist pattern 210. Hence, the analyzer cannot measure the amount of shift s in individual photoresist pattern 210 but can only measure the overall deviation of a group of several photoresist lines 210. Because a pair of photoresist pattern 210 flanking a phase shift region 130 moves an identical distance s in opposite directions, overall shifting of the photoresist patterns 210 is zero. Consequently, an overlay error analyzer cannot be used to measure the amount of shifting in photoresist pattern 210 and deduce the phase error of a mask.
Accordingly, one object of the present invention is to provide a phase shift error monitor pattern that can be used to monitor the phase errors in an alternating phase shift mask. The phase shift monitor pattern includes an alternating phase shift pattern and a modification pattern. The alternating phase shift pattern is located at the periphery of the alternating phase shift mask. The modification pattern is located at the periphery of a modification mask. The alternating phase shift mask and the modification mask are sequentially used in the same exposure process. The alternating phase shift pattern includes a plurality of first non-transparent regions and a plurality of phase shift regions and transparent regions alternately positioned between the first non-transparent regions. The first non-transparent regions on each side of a phase shift region form a pair. The modification pattern includes a plurality of second non-transparent regions. Each second non-transparent region in the modification pattern corresponds to a pair of first nontransparent regions on a first side. The width of each second non-transparent region is larger than the width of any one of the first non-transparent regions.
This invention provides a method for monitoring the phase error of an alternating phase shift mask. The method makes full use of the phase error monitor pattern of this invention. First, a photo-exposure of a positive photoresist layer is conducted using a light source and an alternating phase shift mask having an alternating phase shift pattern thereon (a portion of the phase shift error monitor pattern). Hence, an unmodified photoresist pattern is formed on the positive photoresist layer. The various photoresist patterns correspond to various first non-transparent regions. In addition, the photoresist patterns can be grouped together into a plurality of pairs with each pair corresponding to a pair of first non-transparent regions. A photo-exposure of the positive photoresist layer is again conducted using a modification mask having a modification pattern thereon (another portion of the phase shift error monitor pattern). Hence, every pair of photoresist patterns on the first side is removed while every pair of photoresist patterns on the second side is retained. Ultimately, a monitor photoresist pattern is formed. Thereafter, an overlay error analyzer is used to measure the amount of deviation in this monitor photoresist pattern. Finally, phase error of the alternating phase shift mask is deduced from the defocusing value of the light source and the measured amount of deviation of the monitor photoresist pattern.
This invention also provides a method of monitoring the positional error of a photoresist pattern through an alternating phase shift mask. The method includes using the alternating phase shift mask to perform a photo-exposure of a photoresist layer. Thereafter, a photo-exposure of the photoresist layer is conducted using a modification mask so that every pair of photoresist patterns on a first side are removed while every pair of photoresist patterns on a second side are retained. Finally, an overlay error analyzer is used to measure the amount of deviation of the photoresist patterns remaining on the second side.
The monitor photoresist pattern produced by the phase error monitor pattern of this invention retains only every pair of photoresist patterns on the same side. Since each one of these retained photoresist patterns shift an identical distance towards one direction, overall amount of shifting for these photoresist patterns (in other words, the amount of shifting in the monitor photoresist pattern) is non-zero. Ultimately, an overlay error analyzer is able to measure the amount of shifting and deduce the phase error in the alternating phase shift mask from the measurement.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.